In a large-scale network, a system to remotely monitor each node connecting to the network is indispensable.
As a configuration to transmit a monitor signal, a control signal, and a status signal (generically referred to hereinafter as a “monitor control signal”) between a monitor center and each node, one typical type is to multiplex a monitor control signal on a network to transmit a data signal itself, hereinafter referred to as a “signal network”, and the other is to provide another external network besides the signal network to transmit a monitor control signal through the external network. Such external networks used as a monitor control network comprise for example public networks and/or private networks.
In an optical network to transmit a WDM optical signal of 10 Gb/s per wavelength, for example, a monitor control signal is 2 Mbit/s or 155 Mbit/s.
Generally, each node comprises a cross connector or an add/drop multiplexer to transmit a signal with another node and with a transceiver terminal, and a controller having functions to transmit/receive a monitor control signal and to control the cross connector or the add/drop multiplexer according to the monitor signal or the control signal.
In one configuration, a controller disposed on a specific node functions as a monitor center, and in the other configuration, a monitor center is provided besides a controller disposed on each node. In the latter, one possible case is to connect the monitor center with the controller of each node and the other possible case is to connect the monitor center with a controller of a specific node and to transmit/receive a monitor control signal to/from controllers of the other nodes through the controller of the specific node.
Main four roles of the monitor control signal are (1) monitoring an alarm signal generated from each part and informing the monitor center when an alarm signal is detected, (2) monitoring continuity and normality of signal transmission between adjacent nodes, (3) crossconnect-controlling (provisioning) of an optical path, and (4) signaling about relief (protection/restoration) protocol when any optical path has a fault.
A fault in the monitor control network gives a bad influence to the operation and maintenance of the signal network. Accordingly, the monitor control network needs to have the durability for fault and reliability equal to or even more than those of the signal network.
When an external network is used as the monitor control network, the reliability of the monitor control network depends on that of the external network to be used. Furthermore, the monitor control signal depends on the paths, the path control, the bandwidth, and the state of convergence of the external network to be used. Therefore, such an external network is unsuitable for a swift supervision/control because not only it has a long delay time but also it is unstable.
On the other hand, when a signal network is used as the monitor control network, namely in a case that a monitor control signal is transmitted through a signal network, a fault of the signal network becomes exactly a fault of the monitor control network. When fault occur in a plurality of links, several nodes are isolated in consequence. In such case, it is impossible to monitor and control the isolated nodes. Generally, a monitor control signal is assigned one wavelength and therefore it also leads to the waste of resources.